The invention relates to a motor of the type having its rotor surrounded by a tubular housing and having this housing in turn surrounded by the stator. Such motors are used for driving a pump for circulating a liquid which must be hermetically sealed from the ambient, for example to prevent the liquid from creating dangers to safety and health, and which must also be protected from any contact with liquid-polluting elements such as common lubricating oil. It is usual in the circulating of such a liquid to use portions of the circulated liquid itself both as coolant and as lubricant for the motor driving the liquid circulating pump and for this purpose particularly to enclose the liquid and the rotor of the motor in a hermetically sealed system of enclosures or housings. The housing for the rotor off the pump actuating motor is sometimes called a gap tube, and is commonly as a tube providing a narrow, liquid filled gap between the rotor and the inside of the tube. The stator of the motor surrounds the tube and is sealed off thereby from contact with the liquid, to protect the stator and also the liquid.
Many of the liquid media and particularly many of the cooling and lubricating liquids circulated in systems of the indicated type tend to become viscous or even to congeal below certain temperatures, often including the common room or ambient temperature. In systems of this type, difficulties are encountered at the time of restarting the unit after a shutdown, because of the greatly increased torque which acts on the rotor as a result of the viscous or congealed condition of the liquid wherein the rotor is immersed. Attempts have therefore been made to liquefy congealed and viscous liquid in the rotor chamber, by application of heat at times between the periods of normal operation of the motor.
Small motors can be heated for such purposes by repeated starting and stopping of the motor, but this process is time consuming and is therefore not acceptable where the motor-pump unit is for example, a reserve unit, which must be started without delay. In the case of relatively large motors the repeated starting and stopping would also lead to structural damage and would be unacceptable for this reason.
Other attempts have therefore been made wherein an entire motor was heated from the outside preliminary to its restarting, for example by heating conduits wherein steam or hot water circulated, or by electric heating elements. This system is expensive since it must heat the entire mass of the motor in order to heat the relatively small mass of liquid in the gap tube.
Still another, rather theoretical possibility has been considered, in which an electric heating current is passed through the stator coils. This is another method which involves rather large cost. If direct current be passed through the stator for the indicated purpose, heating is effected only by the resistance of the stator coils; for this reason, high densities of current are needed which are all too expensive. If two-phase alternating currents are used for heating the motor through the coils, unproportionally large transformers are needed due to the resulting poor power factor. Another drawback of the method of heating by current passing through the stator coils is that too much of the generated heat is lost to the ambient, unless the entire motor be encased in an unwieldy and expensive heat-insulating casing.
Still another solution has been proposed: to feed specially heated liquid into the rotor space. This requires special valves and valve operations and corresponding expenses and losses of time. Therefore, this method is warranted at most at the time of first operation of a motor pump; it is not acceptable in other cases and mainly not in cases where the motor pump unit must be in readiness promptly, for example to operate instead of a disabled pump.